I'd also add a vapor barrier to reduce humidity in the crawlspace. 

So, I have a somewhat working knowledge of physical plant issues (sagging roofline, foundation issues, vapor and moisture problems, etc.)

If one or more people are sleeping in the room then the problem is most-likely cause by the vapor the person(s) are breating.As mentioned in several posts, my company deals with your type of problem and most-often the problem is the result of the people breathing vapors when they sleep and lack of good air movement and ventilation in the room.

It's a grind and my savings is on vapors. 

Now my war chest is on vapors and I only have the "free" options for marketing now, and brokerages in my case say "just call people you know". 

If I just do no cell spray foam, will I create a vapor issue?

If you really want to renovate NOW then offer her money to leave, otherwise I would let her be and terminate at the end of her lease. 

The more into this process I get, the more questions I have that I’ve been struggling to get answered or have been nervous to ask my realtor about due to her being a family friend so I was hoping I could get them answered here:1.

@Angel Gonzalez and @Herb Dorn  I agree with @Bill Exeter's note of caution. 

My question is if I went in with my sister as a co borrower (her being the occupant) could the title be switched solely to me after a year or two when I’m ready to develop it?